The present invention relates to a method for operating a hybrid drive and such hybrid drive.
Hybrid drives for driving movable elements of machine tools, in particular also of injection molding machines, are generally known. For example, it is known to provide a drive, for example for an axial feed, composed of both an electric and a hydraulic partial drive for driving the plasticizing and injection unit of an injection molding machine. Reference is hereby made to DE 43 44 335 C2, wherein in one embodiment, the driving force produced by a combination of electric motors is hydraulically supported.
A similar device of a combination of hydraulic with an electric drive for the axial feed is known from JP 04189525.
In addition, reference is made to EP 760 277 which discloses a control method for the hydraulic support of an electric drive, wherein pressure is applied to a hydraulic piston via a storage device for a pressurized medium when a defined control valuable that corresponds to a defined load state of the electric motor has been reached. The pressure increase in the cylinder is hereby proportional to the load on the electric motor.
DE 101 04 109 A1 discloses another control method for the hydraulic support of an electric drive for an axially movable machine part in an injection molding machine. Both the force from an electric motor, wherein the rotation of the electric motor is converted by a gear into a longitudinal motion, and the force from the piston of a hydraulic cylinder operate on the axially movable machine part in the longitudinal direction. The force applied by the electric motor is limited to a value that prevents damage to the gear. The force component generated by the electric motor is limited by forming a control deviation from the desired value for the force operating in the axial direction on the machine element and the actual value of this force, with the control deviation controlling the sum of the force portions operating on the machine element in the axial direction. A desired value for the force operating in the axial direction on the piston is formed from the actual value of the force operating in the axial direction on the machine element and a value which takes into account the mechanical loading capacity of the gear. A control deviation is formed from the desired value for the force operating in the axial direction on the piston and its actual value, which controls a force portion operating on the machine element in the axial direction.
Disadvantageously, the hydraulic cylinders are typically controlled with proportional valves which determine the forward and backward movement commensurate with their control. Such control ensures that the movement in a predetermined application occurs at a predetermined speed. The speed with which the hydraulic cylinder moves is here proportional to the volume flow. A volume flow of different size can be produced by deflecting the valve more or less. On the other hand, a less rapid displacement of the cylinder also corresponds to a higher degree of throttling during the flow through the valve, which disadvantageously converts excess energy that would otherwise be available into heat which is then lost.